Osmosis is a known phenomenon in which water moves across a semi-permeable membrane between solutions with lesser and greater concentrations. In forward osmosis (‘FO’), where the hydraulic pressure difference of the solutions is zero, the water moves from the lower concentration solution to the higher concentration solution. In pressure-retarded osmosis (‘PRO’), where hydraulic pressure is applied to the higher concentration solution in an amount which is greater than zero and lower than the osmotic pressure difference of the solutions, the rate of flux of water can be reduced. If sufficient pressure is applied to the higher concentration side, which is greater than the osmotic pressure differences of the solutions, osmotic water flow can be reversed, referred to as reverse osmosis (‘RO’) and water caused to move across the semi-permeable membrane from the higher to the lower concentration solution (flux reversal point). These techniques have found use in a number of fields, including water treatment and desalination. Pressure-retarded osmosis has also been applied in power generation, where the osmotic pressure difference between seawater or concentrated brine, and fresh water is converted into hydrostatic pressure in a saltwater solution, and the increased hydrostatic pressure is used to drive a turbine. The general equation describing water flux in osmotic-driven membrane process in FO, RO and PRO is Jw=A(σΔπ−ΔP) where Jw is the water flux, A the water permeability constant of the membrane, σ the reflection coefficient, and ΔP is the applied pressure. For FO, ΔP is zero, for RO, ΔP>Δπ and for PRO, Δπ>ΔP.
One common element for all of the above mentioned osmotic technologies is the use of a semi-permeable membrane which allows the passage of water across it but rejects most solute molecules or ions. A persistent problem with known osmosis systems is that of fouling of the semi-permeable membrane. Membrane fouling refers to the potential deposition and accumulation of constituents in the feed stream on the membrane surface and it is usually classified into four major types: colloidal fouling, organic fouling, inorganic fouling/scaling and biofouling. Membrane fouling is a global issue, which limits the membrane operating flux, increases power consumption and requires periodical membrane cleaning-in-place (CIP) procedures. This may result in low effectiveness and high cost, and adds environmental issues related to the CIP chemical solutions disposal. Various preventing and cleaning strategies have been developed based on the understanding of the different factors affecting the fouling process. There is a consensus that membrane cleaning is the long term solution to remove foulants and maintain membrane performance. Cleaning methods include chemical and physical methods. Chemical cleaning is used more widely, however it has huge disadvantages due to system down time which stops production, high costs, environmental issues related to waste chemical disposal and it reduces membrane life time.
For the last few decades pressure-driven processes, such as RO, have been dominant. Several emerging applications based on FO may outperform RO both economically and environmentally. Among them PRO which is a derivative process of FO and may provide a clean and renewable source of energy. FO uses the osmotic pressure gradient (Δπ) to drive water transport through the membrane. In reality, the effective Δπ across the membrane is much lower than the bulk osmotic pressure differences due to membrane orientation and membrane-associated transport phenomena—external and internal concentration polarization. External concentration polarization (ECP) has a single component in pressure-driven membrane processes, referred to as concentrative ECP and is located on the feed side of the active layer. During osmosis-driven membrane process concentrative ECP is followed by dilutive ECP. Both concentrative and dilutive ECP phenomena reduce the effective, net, osmotic driving force. FO is characterized in relatively low permeate flow and therefore the effect of external polarization is relatively low.
The semi-permeable membrane is asymmetric and has a dense active ion-rejecting layer supported by a porous layer. When a draw solution is against the active layer there is only ECP. However when the draw solution is against the porous supporting layer, a dilutive internal concentration polarization (ICP) occurs. An opposite membrane orientation, in which draw solution is placed against the active layer and feed solution against the porous support layer a concentrative IPC occurs. The effect of ICP is detrimental and it reduces the effective, net, driving force between the two solutions.
As stated above, fouling process is a multi-factorial process. The flow configuration of the membrane process may also affect the fouling process. There are two main flow configurations of membrane processes: cross-flow and dead-end filtration. In cross-flow filtration the feed flow is tangential to the surface of membrane, while permeate is directed normal to the membrane surface. In dead-end filtration the direction of the fluid flow is normal to the membrane surface. Dead-end filtration is usually a batch-type process, where all the filtering solution is fed into a membrane device, which then allows passage of some particles subject to the driving force. The main disadvantage of dead end filtration is the extensive membrane fouling and concentration polarization. The fouling is usually induced faster at higher driving forces and water flux. The unidirectional characteristic of dead-end filtration lacks any internal membrane cleaning effects and it comes to a complete stop once the membrane is fully clogged. Tangential flow devices are more cost and labour intensive, but they are less susceptible to fouling due to the sweeping effects and high shear rates of the passing flow. In an RO process the feed goes through a cross flow configuration while in FO and PRO the draw solution goes through a cross flow configuration and the feed solution passes through dead-end filtration. As such, at high feed stream, a PRO system would be very prone to dead-end fouling effects and due to cleaning requirements will have to work in batches with prolonged down times.
U.S. Pat. No. 7,658,852 to Liberman teaches an on-line direct osmosis cleaning waves by discharging pulses of high salinity solution (‘DO-HS’) along the feed water stream in an RO system as a better alternative to known CIP processes. DO-HS cleaning does not interrupt the operational process of the system. The cleaning wave reverses locally the RO process into a FO process and effectively activates four synergetic cleaning effects within a short time frame of about 20 seconds: (1) fouling lifting; (2) fouling sweeping; (3) bio-osmotic shock; and (4) salt dissolve shock. The cleaning wave creates a local effect of backwash stream through the membrane by instantaneously switching the cross flow into a dead-end flow. This local effect propagates in a wave pattern along the membrane so that the entire membrane is cleaned.
U.S. Pat. No. 4,033,878 to Foreman and U.S. Pat. No. 8,354,026 to Herron teach PRO systems which use proprietary membrane structure and system design. Amongst other things, these patents teach a system which allow cross flow configuration both for the draw solution and the feed solution. In addition, special spiral membrane design is required to allow FO to take place as standard RO spiral membrane structure does not allow feed solution in FO to flow in the envelop. These patents do not teach any cleaning effects and therefore are susceptible to system down time for cleaning and maintenance.
There is a need to develop a PRO system which can practice standard spiral membranes and can work continuously with minimal interruptions based on reliable internal cleaning process to minimize the down time periods and maximize efficiency.